Of recent years, some exhaust emission control devices in, for example, exhaust system for diesel engine have muffling structure. An example of such exhaust emission control device is shown in FIGS. 1-3. In the figures, reference numeral 1 denotes an exhaust emission control device. The exhaust emission control device 1 comprises a muffling structure 2 arranged upstream in a direction D of gas flow, an oxidation catalyst 3 arranged downstream of the muffling structure 2 in the direction D of gas flow and a particulate filter 4 arranged downstream of the catalyst 3 in the direction D of gas flow. The muffling structure 2, the oxidation catalyst 3 and the particulate filter 4 are encased in a cylindrical casing 5.
The oxidation catalyst 3 is carried by, for example, a cordierite or other ceramic carrier which provides a flow-through type honeycomb structure. The particulate filter 4 comprises, for example, a filter body (not shown) in the form of a porous honeycomb structure made of ceramic such as cordierite and having lattice-like compartmentalized passages. Alternate ones of the passages have plugged inlets and the remaining passages with unplugged open inlets are plugged at their outlets. Thus, only the exhaust gas passing through the thin porous compartment walls is discharged downstream and particulates are captured on inner surfaces of the compartment walls.
The muffling structure 2 has a dispersion chamber in the casing 5 upstream in the direction D of gas flow and having a dispersion plate 6 intermediately in the chamber in the direction D of gas flow. Thus, the dispersion chamber is divided into dispersion chambers 7a and 7b upstream and downstream in the direction D of gas flow, respectively. Inserted from outside of and into the dispersion chamber 7a upstream in the direction D of gas flow is an inner pipe 8 which has a tip end fixed to the dispersion plate 6.
A portion of the inner pipe 8 within the dispersion chamber 7a is formed with a plurality of smaller- and equi-diameter pores 9 spaced apart from each other circumferentially and in the direction D of gas flow with predetermined pitches. The dispersion plate 6 facing the chambers 7a and 7b is formed with a plurality of circumferentially equidistant larger-diameter pores 10 in a circle, and is also formed with a plurality of pores 11 and 12 each having a diameter smaller than that of the pore 10 and in circles different from that in which the pores 10 are arranged. The dispersion plate 6 is further formed with a plurality of pores 13 with a diameter smaller than that of the pores 10 and at a position corresponding to an inner diameter region of the inner pipe 8. The dispersion plate 6 may be formed with pores in various other patterns.
The exhaust gas from the engine into the inner pipe 8 in the muffling structure 2 partly flows via the pores 9 into the dispersion chamber 7a and thus into the dispersion chamber 7b via the pores 10, 11 and 12 on the dispersion plate 6 and remaining part of the exhaust gas flows via the pores 13 on the plate 6 into the dispersion chamber 7b. This muffles the exhaust gas flow.
The exhaust gas thus flowing into the dispersion chamber 7b passes through the oxidation catalyst 3 into the particulate filter 4 where the exhaust gas is freed from emissions such as particulates and is discharged downstream. In this situation, if the temperature of the exhaust is lower than a predetermined temperature, fuel (HC) for addition is added into the exhaust system upstream of the inner pipe 8 in the direction D of gas flow and the added fuel brings about oxidation reaction on the oxidation catalyst so that the resultant reaction heat elevates in temperature the exhaust; as a result, the emissions such as particulates captured by the particulate filter 4 in clogging manner are burned out by the heat, thereby regenerating the particulate filter 4.
Patent Literature 1 is a prior art reference which discloses an exhaust emission control device with a muffler.    [Patent Literature 1] 2004-263594A